This invention relates to temperature sensors, specifically shape memory alloy temperature sensors that provide persistent indication once their temperature reaches a critical value.
Exposure to temperatures above a critical temperature can damage many important materials. Food products such as frozen dairy products and frozen meats can spoil when exposed to thawing temperatures for even a short time. Frozen medical products such as blood and certain pharmaceuticals can be unsafe once exposed to thawing or other high temperatures, even if the temperature later returns to a safe value. Low temperatures can also compromise important properties of some rubber and rubber-like materials. The damage is often unseen, and can persist even if the temperature returns to an acceptable level. This situation can arise in transportation, where a frozen product temporarily experiences high temperatures due to improper handling or cooling equipment malfunction.
Many conventional temperature sensors do not provide a persistent record of temporary temperature deviations. Conventional temperature sensors, such as common thermometers, indicate the current temperature only. They provide a continuous indication of the current temperature of the material. They do not provide a permanent indication of out-of-range temperatures without additional permanent recording apparatus. Accordingly, there is a need for sensors that provide a persistent record of temporary out-of-range temperatures.
Shape memory alloys (SMAs) have properties that might be useful in developing the needed sensors. An SMA has a certain shape, its Austenitic state, at temperatures below the SMA""s Austenitic temperature Af. The SMA moves in a certain fashion to a second shape, its Martensitic state, when the temperature rises above the Austenitic temperature Af. The SMA will not return to the Austenitic shape without additional external force even if the temperature subsequently falls below the Austenitic temperature Af. SMAs are used in a variety of applications, such as those described in xe2x80x9cDesign and Modeling of a Novel Fibrous SMA Actuator,xe2x80x9d Proc. SPIE Smart Materials and Structures Conference, vol. 2190, pp. 730-738 (1994), and xe2x80x9cA Phenomenological Description of Thermodynamical Behavior of Shape Memory Alloys,xe2x80x9d Transactions of the ASME, J. Appl. Mech., vol. 112, pp. 158-163 (1990). SMAs have been suggested for use in persistent temperature indicators. See Shahinpoor, U.S. Pat. No. 5,735,607, incorporated herein by reference. The sensors suggested by the U.S Pat. No. 5,735,607 however, can require that the apparatus be kept below the threshold temperature during assembly and storage. This requirement can complicate manufacture and handling. There is a need for temperature indicators that can be manufactured, stored, and handled at arbitrary temperatures, then enabled to provide a persistent record of temporary temperature deviations.
The present invention comprises a sensing element mounted with a body. The sensing element comprises a portion made with a shape memory alloy. The sensing element mounts with the body, fixedly at two ends. The fixed mounting at one end, however, is configured so that it can be converted to allow a moveable relationship between the body and the sensing element. While the mounting is fixed, the sensing element is prevented from indicating temperature deviations, and the apparatus can be stored and handled freely. After the mounting is converted to provide a moveable relationship, however, temporary excursion can cause the SMA portion to contract, providing a discernible change in the relationship between the sensing element and the body.
The SMA portion can be a wire made with a SMA, of length equal to the distance between the two mounting portions in one state, and less than the distance in the other state. The wire can be passed through an opening such as a notch or hole in the body, and a bend or a thickened portion used to prevent the wire from moving back through the opening. After the apparatus is in place, and the temperature reduced below the critical temperature, then the bend or thickened portion can be removed. Subsequent temperature increases can initiate a shortening of the SMA wire; the difference in length relative to the body can provide an indication of an over-temperature condition. Even if the temperature excursion was only transitory, the SMA wire will not appreciably lengthen once shortened, absent application of external force. Discernment of the condition of the apparatus can be facilitated by highlighting the relationship between the sensing element and the body. As examples, colored regions and witness marks can be used to make changes in the length or shape of the SMA portion readily apparent.
Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.